1. Field of the Invention
The present invention relates to a display device, and more particularly, to an electrically-driven liquid crystal lens, which can achieve not only a gentle parabolic lens plane when being realized via alignment of liquid crystals based on a changed electrode configuration, but also a reduced cell gap of a liquid crystal layer and a stable profile even in a large-area display device, and a stereoscopic display device using the same.
2. Discussion of the Related Art
At present, services for rapid dissemination of information, constructed on the basis of high-speed information communication networks, have developed from a simple “listening and speaking” service, such as current telephones, to a “watching and listening” multimedia type service on the basis of digital terminals used for high-speed processing of characters, voice and images, and are expected to be ultimately developed into cyberspace 3-dimensional stereoscopic information communication services enabling virtual reality and stereoscopic viewing free from the restrains of time and space.
In general, stereoscopic images representing 3-dimensions are realized based on the principle of stereo-vision via the viewer's eyes. However, since the viewer's eyes are spaced apart from each other by about 65 mm, i.e. have a binocular parallax, the left and right eyes perceive slightly different images due to a positional difference therebetween. Such a difference between images due to the positional difference of the eyes is called binocular disparity. A 3-dimensional stereoscopic image display device is designed on the basis of binocular disparity, allowing the left eye to view only an image for the left eye and the right eye to view only an image for the right eye.
Specifically, the left and right eyes view different 2-dimensional images, respectively. If the two different images are transmitted to the brain through the retina, the brain accurately fuses the images, giving the impression of real 3-dimensional images. This ability is conventionally called stereography, and a stereoscopic display device is obtained by applying stereography to a display device.
A stereoscopic display device may be classified based on constituent elements of a lens which realizes 3-dimensional images. As one example, there is an electrically-driven liquid crystal lens wherein a liquid crystal layer constitutes a lens.
Generally, a liquid crystal display device includes two electrodes opposite each other, and a liquid crystal layer interposed between the two electrodes. Liquid crystal molecules of the liquid crystal layer are driven by an electric field created when voltages are applied to the two electrodes. The liquid crystal molecules have polarization and optical anisotropy characteristics. With polarization, when liquid crystal molecules are under the influence of an electric field, electric charges in the liquid crystal molecules are gathered to opposite sides of the liquid crystal molecules, whereby a molecular arrangement direction is altered according to the electric field. With optical anisotropy, owing to an elongated shape of liquid crystal molecules and the above-mentioned molecular arrangement direction, the path or polarization of light to be emitted is changed according to the incidence direction or polarization of incident light.
Accordingly, the liquid crystal layer has a difference in transmissivity by voltages applied to the two electrodes, and an image can be displayed using the transmissivity difference of pixels.
Recently, there has been proposed an electrically-driven liquid crystal lens wherein a liquid crystal layer serves as a lens using the above-described characteristics of liquid crystal molecules.
Specifically, a lens is designed to control the path of incident light on a per position basis using a difference between an index of refraction of a lens constituent material and an index of refraction of air. In the electrically-driven liquid crystal lens, if different voltages are applied to the liquid crystal layer according to different positions of electrodes so as to drive the liquid crystal layer by different electric fields, incident light introduced into the liquid crystal layer causes different phase variations on a per position basis and as a result, the liquid crystal layer can control the path of incident light in the same manner as an actual lens.
Hereinafter, a related art electrically-driven liquid crystal lens will be described with reference to the accompanying drawings.
FIG. 1 is a sectional view illustrating a related art electrically-driven liquid crystal lens, and FIG. 2 is a view illustrating electric potential distribution upon formation of the electrically-driven liquid crystal lens of FIG. 1 after voltages are applied to the liquid crystal lens.
As shown in FIG. 1, the related art electrically-driven liquid crystal lens includes first and second substrates 10 and 20 opposite each other, and a liquid crystal layer 30 formed between the first substrate 10 and the second substrate 20.
Here, first electrodes 11 are arranged on the first substrate 10 with a first interval. In this case, with relation to the neighboring first electrodes 11, a distance from the center of one of the first electrodes 11 to the center of the next first electrode 11 is called “pitch”. Repeating the same pitch for the respective first electrodes results in a pattern.
A second electrode 21 is formed over the entire surface of the second substrate 20 opposite the first substrate 10.
The first and second electrodes 11 and 21 are made of transparent metals. The liquid crystal layer 30 is formed in a space between the first electrodes 11 and the second electrode 21. Liquid crystal molecules of the liquid crystal layer 30 respond to the strength and distribution of an electric field and have a phase distribution similar to that shown in FIG. 2.
The above-described electrically-driven liquid crystal lens is realized based on an assumption that high voltages are applied to the first electrode 11 and the second electrode 21 is grounded. With this voltage condition, a vertical electric field is strongest at the center of the first electrode 11, and the strength of the vertical electric field decreases away from the first electrode 11. Accordingly, when the liquid crystal molecules of the liquid crystal layer 30 have positive dielectric constant anisotropy, the liquid crystal molecules are arranged according to the electric field in such a way that they are upright at the center of the first electrode 11 and are gradually tilted horizontally away from the first electrode 11. As a result, in view of light transmission, an optical path is shortened at the center of the first electrode 11, and is lengthened with increasing distance from the first electrode 11, as shown in FIG. 2. Representing the length variation of the optical path using a phase plane, the electrically-driven liquid crystal lens has light transmission effects similar to a parabolic lens.
Here, the second electrode 21 causes the behavior of an electric field, making an index of refraction generally take the form of a spatial parabolic function and the first electrodes 11 define edge regions of the lens.
In this case, relatively high voltages are applied to the first electrodes 11 than the second electrode 21. Therefore, as shown in FIG. 2, an electric potential difference occurs between the first electrodes 11 and the second electrode 21. In particular, a steep horizontal electric field is created around the first electrodes 11. Accordingly, liquid crystals molecules have a slightly distorted distribution rather than a gentle distribution, whereby an index of refraction cannot exhibit parabolic spatial distribution, or movement of the liquid crystals is excessively sensitive to voltage variation.
The above-described related art electrically-driven liquid crystal lens can be realized, without a physical parabolic lens, by arranging electrodes on two substrates with liquid crystals interposed therebetween and applying voltages to the electrodes.
However, the above-described electrically-driven liquid crystal lens has the following problems.
First, since electrodes formed on a lower substrate occupy an extremely partial area of a lens region, a steep horizontal electric field, rather than a gentle electric field, is created between a lens edge region corresponding to the electrodes and a lens center region distant from the lens edge region, resulting in a slightly distorted phase of the electrically-driven liquid crystal lens. In particular, in the electrically-driven liquid crystal lens wherein high voltages are applied to a limited number of electrodes in each lens region, the greater the pitch of the lens region, an insufficient electric field is created between the high voltage electrodes and substrates opposite each other. Accordingly, formation of the electrically-driven liquid crystal lens having a gentle parabolic lens plane having the same optical effects as an actual lens is extremely difficult.
Second, when being applied to a large-area display device, the lens center region, which is distant from the lens edge region where the electrodes, to which a high-voltage is applied, are located, is unaffected by an electric field and has difficulty in alignment control of liquid crystals by the electric field. This causes a serious distortion in lens shape based on the electric field. As occasion demands, when control in the lens center region is difficult or impossible, the resulting electrically-driven liquid crystal lens has a discontinuous lens profile and is ineffective as a lens.